Something like adding a value to an area of an image

Well, first I advise you to learn how to draw using GDI+, something that isn't too hard to learn. Displaying data from a database when you click on a room, however, requires you to be able to connect raw mouse coordinates with a room, which is a lot more work and probably not a good idea to attempt if you don't already have a vague idea of how to do it, especially if the room is not a rectangle. Basically there are no pre-made classes to help you with the floorplan part beyond drawing it so you will have to write your own.